I. Field of the Invention
The present invention relates to a surface acoustic wave material and device and, more particularly, to a surface acoustic wave material and device having a high electromechanical coupling coefficient.
II. Description of the Prior Art
In order to achieve a compact surface acoustic wave device having a low insertion loss and a wide band, a piezoelectric substrate having a high electromechanical coupling coefficient is the most important requirement. Conventionally, a 128.degree. lithium niobate rotated Y-cut crystal for Rayleigh waves, an SH (shear horizontal wave) type 41.degree. lithium niobate rotated Y cut crystal for leaky surface waves, and SH type 36.degree. and 64.degree. lithium niobate rotated Y-cut crystals for leaky surface waves were proposed in the paper "Propagation and amplification of Rayleigh waves and piezoelectric leaky surface waves in LiNbO.sub.3 ", by K. Yamanouchi et al (J.A.P., 43, P. 856-862, 1972) and in the paper "Some Simple Modes of Wave Propagation in an Infinite Piezoelectric Plate", by J. L. Bleustein (J., Acoust, Soc. Amer., 45, P. 614-619, 1969). However, the 128.degree. lithium niobate rotated Y-cut crystal and the 36.degree. lithium tantalate rotated Y-cut crystal described above have coupling coefficients of only 5.5% and 5%, respectively. The 41.degree. lithium niobate rotated Y-cut crystal and the 64.degree. lithium niobate rotated Y-cut crystal described above have coupling coefficients of 17.2% and 11.3%, respectively, thus attaining relatively high values. However, an abrupt propagation loss occurs in these crystals due to a shift in energy flow angle. This propagation loss is a characteristic problem.